Phase inversion circuit



July 7, 1942. A. w. BARBER 2,289,301

PHASE INVERS ION CIRCUIT Filed Jan. 26, 1939 IN VENTOR Patented July 7, 1942 UNITED STATES PATENT OFFICE 2,289,301 7 PHASE INVERSION CIRCUIT Alfred W. Barber, Flushing, N. Y. Application January 26, 1939, Serial N0. 252,900

Claims.

My present invention relates to thermionic vacuum tube amplifier systems. In particular it relates to an improved system for generation of split phase voltages in audio amplifiers and the like.

One object of my present invention is to provide apparatus for generating equal and oppositely phased voltages from a single phased input voltage; A particular object is to provide apparatus and circuits which reduce the distortion generated in such circuits and to provide means for automatically maintaining the balance in such circuits.

In the past several methods have been employed to generate equal and oppositely phased voltages suitable for actuating push-pull circuits from a single phase of input voltage. One method is by means of a center-tapped transformer secondaryfed with a single phased primary voltage. Another method is to apply a portion of the output of one tube to the grid of a secondtube in such a manner that the alternating plate voltages of the two tubes are equal and out of phase.

Still another method is to use the alternating current plate and cathode voltages of a single tube fed with a single phase of grid voltage as equal and out of phase voltages.

My present invention employs the cathode voltage generated by the plate current of a first or input tube applied to the cathode of the second or reversed phase tube to produce phase inversion. In order to produce balanced output voltages from the two tubes a somewhat lower plate load resistor or impedance is used in the input tube plate circuit than in the reversed phase tube plate circuit. Two tubes may be employed or a double type tube having two sets of cold electrodes associated with a common cathode. The system is degenerative to some degree Which'reduces distortion and the balance in the system is automatically maintained. The operation of the system may be more fully understood by following the following description when taken in connection with the drawing.

In the drawing:

Fig. 1 shows one embodiment of my invention employing two tubes.

Fig. 2 shows another embodiment of my invention employing a double type tube.

Fig. 3 shows still another embodiment of my invention in a direct coupled system. The circuit of Fig. 1 shows a source of signal I which may be a radio receiver detector, microof single phase voltage which is to be split or inverted to produce. a two phase balanced output. The thermionic vacuum tubes 2 and 3 are employed to amplify the input signal and supply a second phase of voltage. The first or input tube [is shown as a triode having a control grid 4; cathode 5 and plate 6 although a tetrode or pentode may be used as well. The second or inverter tube 3 is also shown as a triode having a control grid 1, cathode 8 and plate 9 although it too may be another type such as a tetrode or pentode. Both cathodes 5 and 8 are connected to ground G or the low potential side of the system thru the common unby-passed resistor in. Plate 6 of the input tube is connected thru a plate load resistor H, which may-be a general impedance as well, to the positive side of a plate voltage supply which is represented by battery l2. The negative side of supply I! is connected to ground G or the low potential side of the system. Plate 9 of the inverter tube is connected thru resistor l3, which may also be a general impedance,-to the positive side of the plate supply l2. Grid! of the second or inverter tube is grounded or connected to the low potential side of the system. A condenser l4 may be connected across the plate supply l2 to reduce its alternating current impedance. The single phase input voltage from source I is applied between'grid 4 of the input tube and ground G, or the low potential side of the system. One phase of output is taken off between plate 6 and ground G and a reversed phase output is taken oif between plate 9 and ground G. With proper values of resistors I0, II and I 3 the two output voltages will be 180 degrees out of phase and of equal magnitude.

The theory of operation of Fig. 1 will now be set forth. The voltage applied to grid 4 causes an alternating current to flow thru plate resistor II and cathode resistor H). The resulting alternating current voltage drop across resistor I0- is applied to cathode 8 of tube 3 and causes an alternating current to flow in plate resistor I 3 and cathode resistor- I 0. Phases are such that a positively increasing voltage applied to grid 4 will cause plate 6 to become less positive, cathodes 5 and 8 to become more positive and plate 9 more positive thus producing oppositely phased plate Voltages. As far as the input voltage is concerned the current from plate 6 thru resistor phone, phonograph pick-up or iniact any source ill will be degenerative while that from plate 9 will be regenerative. The system operates in such a manner that the degenerative current from plate 6 predominates so that the system embodies degenerative feed-back which reduces the distortion produced by the system. For equal alternating plate voltages the alternating current from plate 6 must be greater than the alternating plate current from plate 9 and plate resistor or impedance H must be smaller than plate resistor or impedance l3.

Mathematically,

is the condition for equal and opposite plate voltages, where:

k is the amplification factor of tubes 2 and 3, r, is the plate resistance of tubes 2 and 3. R1; is the value of resistor l3,

R10 is the value of resistor ID,

R11 is the value of resistor H.

In a typical case tried the constants .were as follows; It equal to 35. Tp equal to 11,000 ohms, R13 equal to 100,000 ohms, R10 equal to 1500 ohms and R11 equal to 88,000 ohms as determined by the tube characteristics and the above formula.

While I prefer to use identical tubes for tubes 2 and 3 this is not required but the mathematics and circuit constants for unlike tubes will be different from those shown above. Tubes 2 and 3 may be combined in a single envelope in the form of a double tube in which two sets of cold electrodes are associated with a common cathode.

In Fig. 2 I have shown a double type tube 23 as the input and inverter tubes utilizing a common cathode 5-43. The function and operation of similarly numbered circuit components is the same as in Fig. 1. The division between plate resistors H and I3 is accomplished by means of a variable tap l5 connected to the positive side of plate supply voltage l2. This variable tap provides means for closely equalizing the two out of phase plate voltages taking into account any differences or changes in tube or circuit constants. The grid 4 of the input sectionof tube 2-3 is shown connected to a slider or tap I1 on the potentiometer l6 connected across the phonograph pick-up 18 or other source of signal. The push-pull output voltages from tube 2-3 are applied to push-pull tubes 24 and 25. Grid 21 of tube 24 is fed from plate 6 thru coupling condenser 19 and across grid resistor 2| connected between grid 21 and ground G. Grid 26 of tube 25 is fed from plate 9 thru coupling condenser and across grid resistor 22 connected between grid 26 and ground G. Since tubes 24 and 25 are operated in push-pull they may use a common cathode resistor 23 connected between cathodes 2S and and ground G. Plates 28 and 3| are connected to the balanced primary of the output transformer 32 which feeds speaker 34. Plate voltage for tubes 26 and 25 is supplied at the transformer primary center tap 33.

One mode of operation of the system of Fig. 2 is to use equal resistors for H and 13 or place tap 15 in the center of the combination in which cases somewhat unbalanced voltages are fed to grids 26 and 21. However, tubes 24 and 25 utilize a common cathode resistor 23 and unequal input voltages are equalized by the resulting amplified signal drop across resistor 23. In this way, sufflciently accurate balance is produced in two steps, first approximately by tube 2--3 and finally more accurately by tubes 24 and 25. This system has been found to be very simple to construct, eflicient in operation, and self-equalizing While tubes 24 and 25 should be as closely matched as possible, any small mis-match may be adjusted for by a variation of tap 15 which adjusts the relative values of resistors H and I3 and hence .the relative magnitudes of the voltages applied to grids 26 and 21. Thus the proper adjustment of tap l5 provides means for balancing the entire system so that it operates at its highest efiiciency and with least distortion.

In Fig. 3 I have shown a modified circuit for feeding push-pull tubes. Coupling condensers and grid leaks are eliminated and grid 26 of tube 25 is fed conductively from plate 9 while grid 21 is fed conductively from plate 6. I1. resistors II and 13 are high resistances plates 6 and 9 will operate at a relatively low potential above ground which permits direct coupling if cathodes 28 and 30 are operated at a voltage greater than normal by the amount of this plate voltage by making cathode resistor 23 larger than normal.

One method of equalizing the direct current voltages on grids 26 and 21 is to connect a resistor 31 from plate 6 to ground G. By mutual adjustment of resistors II and 31 the alternating and direct current voltages on grid 21 may be made equal to those on grid 26. Since load resistor ll may be smaller than resistor 13, the direct current voltage at plate 6 will be higher than at plate 9. Resistor 31 increases the direct current drop in resistor H and if resistor II is made slightly larger than reviously, so that resistors II and 31 in parallel equal the previous signal load of resistor ll, both alternating and direct current voltages on plate 6 may be made equal to those on plate 9.

There are several distinct advantages of my present inverter system over conventional systems. My system employs degeneration which reduces distortion in the inverter stage and still the gain is nearly equal to the stage gain of a single tube. In the case calculated above the gain of a non-degenerative stage would be 31 times while the gain to either plate of my inverter system is 19.5 times which shows a loss of only 4 D. B. in obtaining the improved quality in erted voltages. My system is automatic in that any tendency to unbalance due to aging tubes or other causes is counteracted by the degenerative and regenerative cathode voltages and more accurately balanced output voltages are maintained than in a system where a portion of-the plate voltage of one tube is applied to the grid of the inverter stage without degeneration. To these and other advantages are added advantages of greater circuit simplicity.

While I have shown and described only a fr-w systems whereby my invention may be carried out other modifications are possible within the spirit and scope of the invention as set forth in the appended claims.

What I claim is:

1. In an electrical amplifier, the combination of two similar thermionic vacuum tube amplifiers each including at least a cathode, a control grid and a plate, means for applying a signal to the grid of the first of said tubes, means for grounding the grid of the second of said tubes, a resistance common to both of said cathodes, a load resistance in the plate circuit of said second tube, a

resistance load in the plate circuit of the first said tube approximately equal to the amplification constant of said tubes times the resistance of said cathode resistor times the resistance of the in the presence of changing tube characteristics. plate load of said second tube divided by the sum of the plate resistance of said tubes plus said second tube plate resistor plus said cathode resistance minus the product of said amplification constant and said cathode resistance.

2. In an electrical amplifier, the combination 01 a double type thermionic vacuum tube including a cathode and at least two grids and two plates, means for applying a signal to the first of said grids, means for grounding the second of said grids, a resistance connected between said cathode and ground, a resistance connected to the plate associated with the second said grid and a resistance connected to the plate associated with the first said grid approximately equal to the amplification factor of said tube times theresistance of said cathode resistor times the resistance of said second plate connected resistor divided by the sum of the plate resistance of said tube plus the resistance of the second plate connected resistor plus the resistance of said cathode resistor minus the amplification factor of said tube times the resistance of said cathode resistor.

3. In an electrical amplifier, the combination of a thermionic vacuum tube including at least one cathode, two grids and two plates, means for applying a signal to one of said grids, means for grounding the other of said grids, a resistance path between said cathode and ground, unequal impedances connected between said plates and a source of plate voltage, two additional thermionic vacuum tubes each including at least a cathode, a grid and a plate, a conductive connection between one of the first said plates and one of the second said grids and a conductive connection between the other of the first said plates and the other of the second said grids, a resistor connected between the second said cathodes, a resistor connected between one of the second said cathodes and ground and equal impedances connected between each of the second said plates and a source of plate voltage.

4. In an electrical wave amplifier, two stages of direct-coupled push-pull amplifiers, a source of direct current, unequal loads connected to the first stage of said amplifier and saidsource of direct current, and a resistor connected between the high side of one of said loads and a point of reference potential for increasing the direct current thru said load.

5. In an elctrical amplifier, the combination of, thermionic vacuum tube means including at least two grids, two plates and cathode means common to said grids and plates, means for applying a signal between one of said grids and a point of 'reference, a signal frequency impedance connected between said cathode means and said point of reference, a connection between the second of said grids and said point of reference, and means for developing substantially equal and out-ofphase signals at said plates including unequal im- ALFRED W. BARBER. I 

